Tailoring retained austenite and mechanical properties in super martensitic stainless steel in-situ alloyed via wire-arc directed energy deposition

• In-situ alloying enables tailoring of retained austenite in super martensitic stainless steel. • Compositional gradient governs martensite–ferrite–austenite transitions. • Strength–ductility balance is optimized via phase fractions control. Super martensitic stainless steel (SMSS) is extensively applied in hydroelectric, petrochemical, and nuclear power fields because it combines relatively low cost with excellent mechanical performance. This study innovatively employs in-situ alloying approach to tailor the retained austenite (RA) content in SMSS by adjusting the mixing ratio of austenitic stainless steel (ASS) and ferritic stainless steel (FSS) feedstocks during wire-arc directed energy deposition (DED). The microstructural distribution of the as-deposited component reveals pronounced spatial heterogeneity. With the gradual reduction of γ-stabilizer elements along the building direction (BD), the RA content steadily decreases, whereas δ-ferrite exhibits an opposite increasing trend. Notably, the volume fraction of α’-martensite first increases and then declines. The microstructural evolution correlates directly with mechanical properties, reflected in an initial enhancement followed by a degradation in strength, along with a continuous loss of ductility. Compared with conventional approach, this strategy enables location-specific control of phase fractions and enhances the efficiency and flexibility of designing compositionally graded SMSS components.